Method for dropping packet data, radio communication device, and mobile communication system

ABSTRACT

A mobile communication system, that includes a buffer storing packet data to be sent; and a controller configured for discarding the packet data according to a value of a timer corresponding to the packet data, maintaining the value of the timer corresponding to the packet data when a handover is performed without restarting or resetting the value of the timer, wherein the discarding further includes discarding the corresponding packet data when the value of the timer reaches a given value.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.12/346,020, filed on Dec. 30, 2008, now pending, which is based upon andclaims the benefit of priority of the prior Japanese Patent ApplicationNo. 2008-258, filed on Jan. 7, 2008, the entire contents of which areincorporated herein by reference.

FIELD

The present invention generally relates to a radio communication device,such as a user equipment and/or a base station, and a method fordropping packet data in the radio communication device. The presentinvention is preferably applied to a case where a method for droppingpacket data based on an Active Queue Management (AQM) technology and amethod for dropping packet data based on a timer are used together.

BACKGROUND

Services supported by third generation Code Division Multiple Access(CDMA) systems have been provided for mobile communication systems forcellular phones and so on. Under these circumstances, the 3^(rd)Generation Partnership Project (3GPP) is working on Long Term Evolution(LTE) that achieves a much higher data-rate. Now, 3GPP is confrontedwith hurdles of how to speed up transfer-rates and how to reducetransmission delays.

When a user equipment moves from one location to another location whilein service, a base station that communicates with the user equipmentperforms a handover operation according to the reception condition atthe user equipment. In a hard handover that is mainly applied to apacket switching system, a line between the user equipment and a sourcebase station (handover source base station) communicates with the userequipment until the relocation is disconnected and a line between theuser equipment and a target base station (handover target base station)that is to communicate with the user equipment is connected.

In the hard handover scenario, the user equipment obtains systeminformation on the target base station immediately before the handover.Accordingly, the handover can be carried out in a short time.

On the other hand, an interruption of user data transmission during thehandover causes a transmission delay. Moreover, packet data may be lostin the interruption. In such a case, the lost packet data has to beresent in an end-to-end manner and recovered. This may further increasethe transmission delay.

It would be advantageous to reduce the transmission delay duringhandover in the mobile communication by shortening the transmissioninterruption time during handover and to prevent the packet data loss inthe interruption.

SUMMARY

According to an aspect of the invention, a control method in a mobilecommunication system with a buffer unit for temporarily storing packetdata to be sent, includes dropping the packet data stored in the bufferunit before sending when a drop timer corresponding to the packet databefore sending reaches a given value or when a drop condition requiredby a drop mechanism accompanying the buffer unit is satisfied anddropping the packet data stored in the buffer unit after sending when adrop timer corresponding to the packet data after sending reaches agiven value.

According to an aspect of the invention, a control method in a mobilecommunication system with a buffer unit for temporarily storing packetdata to be sent, wherein the system drops the packet data stored in saidbuffer unit according to a value of a drop timer corresponding to thepacket data, includes changing the value of the drop timer correspondingto the packet data stored in the buffer unit when a handover isperformed, setting the changed value of the drop timer as a new value ofthe drop timer for the packet data stored in the buffer unit, anddropping the corresponding packet data when the new value of the droptimer reaches a given value.

According to an aspect of the invention, a control method in a mobilecommunication system with a buffer unit for temporarily storing packetdata to be sent, wherein the system drops the packet data stored in saidbuffer unit according to a value of a drop timer corresponding to thepacket data, includes taking over the value of the drop timercorresponding to the packet data stored in the buffer unit when ahandover is performed, and dropping the corresponding packet data whenthe taken over value of the drop timer reaches a given value.

According to an aspect of the invention, a control method in a mobilecommunication system with a buffer unit for temporarily storing packetdata to be sent, wherein the system drops the packet data stored in saidbuffer unit according to a value of a drop timer corresponding to thepacket data, includes transferring the packet data from a source basestation to a target base station and transferring the value of the droptimer corresponding to the packet data to be transferred when a handoveris performed, wherein the target base station sets a new value of thedrop timer for the transferred packet data with the transferred value ofthe drop timer or the transferred value of the drop timer corrected witha transfer delay time, and when the new value of the drop timer reachesa given value, drops the corresponding packet data.

According to an aspect of the invention, a control method in a mobilecommunication system with a buffer unit for temporarily storing packetdata to be sent, wherein the system drops the packet data stored in saidbuffer unit according to a value of a drop timer corresponding to thepacket data, includes transferring the packet data from a source basestation to a target base station, and correcting the value of the droptimer corresponding to the packet data to be transferred with a transferdelay time and then transferring the value of the drop timer when ahandover is performed, wherein the target base station sets a new valueof the drop timer for the corresponding packet data with the correctedvalue of the drop timer transferred thereof and when the new value ofthe drop timer reaches a given value, drops the corresponding packetdata.

According to an aspect of the invention, a radio communication device ina mobile communication system with a buffer unit for temporarily storingpacket data to be sent, includes a control unit configured to drop thepacket data stored in said buffer unit before sending when a drop timercorresponding to the packet data before sending reaches a given value orwhen a drop condition required by a drop mechanism accompanying thebuffer unit is satisfied, and to drop the packet data stored in saidbuffer unit after sending when a drop timer corresponding to the packetdata after sending reaches a given value.

According to an aspect of the invention, a radio communication device ina mobile communication system with a buffer unit for temporarily storingpacket data to be sent, wherein the system drops the packet data storedin the buffer unit according to a value of a drop timer corresponding tothe packet data, includes a control unit configured to change the valueof the drop timer corresponding to the packet data stored in the bufferunit when a handover is performed, set the changed value of the droptimer as a new value of the drop timer for the packet data stored in thebuffer unit, and to drop the corresponding packet data when the newvalue of the drop timer reaches a given value.

According to an aspect of the invention, a radio communication device ina mobile communication system with a buffer unit for temporarily storingpacket data to be sent, wherein the system drops the packet data storedin the buffer unit according to a value of a drop timer corresponding tothe packet data, includes a control unit configured to take over thevalue of the drop timer corresponding to the packet data stored in thebuffer unit when a handover is performed, and to drop the correspondingpacket data when the taken over value of the drop timer reaches a givenvalue.

According to an aspect of the invention, a mobile communication systemwith a buffer unit for temporarily storing packet data to be sent,wherein the system drops the packet data stored in the buffer unitaccording to a value of a drop timer corresponding to the packet data,includes transferring means for transferring the packet data from asource base station to a target base station and transferring the valueof the drop timer corresponding to the packet data to be transferredwhen a handover is performed, wherein the target base station includes acontrol unit configured to set a new value of the drop timer for thetransferred packet data with the transferred value of the drop timer orthe transferred value of the drop timer corrected with a transfer delaytime, and to drop the corresponding packet data when the new value ofthe drop timer reaches a given value.

According to an aspect of the invention, a mobile communication systemwith a buffer unit for temporarily storing packet data to be sent,wherein the system drops the packet data stored in the buffer unitaccording to a value of a drop timer corresponding to the packet data,includes a transferring unit for transferring the packet data from asource base station to a target base station, and correcting the valueof the drop timer corresponding to the packet data to be transferredwith a transfer delay time and then transferring the value of the droptimer when a handover is performed. The target base station includes acontrol unit configured to set a new value of the drop timer for thecorresponding packet data with the corrected value of the drop timertransferred thereof and drops the corresponding packet data when the newvalue of the drop timer reaches a given value.

Accordingly, it is an object in one aspect of the invention to reducethe transmission delay in the handover for the user equipment and thebase station.

The objects and advantages of the invention will be realized andattained by means of the elements and combinations particularly pointedout in the claims.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary and explanatory and arenot restrictive of the invention, as claimed.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 depicts an example of a mobile communication systemconfiguration;

FIG. 2 depicts a user plane protocol stack between a base station and auser equipment;

FIG. 3 depicts packet data transfer performed in accordance with ahandover operation;

FIG. 4 depicts an exemplary flowchart for operations of units in theuser equipment upon handover;

FIG. 5 depicts an exemplary flowchart for operations of units in asource base station upon handover;

FIG. 6 depicts an exemplary flowchart for operations of units in atarget base station upon handover;

FIG. 7 depicts an exemplary handover sequence;

FIG. 8A depicts an example of timer-based packet data drop;

FIG. 8B depicts an example of timer-based packet data drop (time 30 haselapsed);

FIG. 9A depicts an example of a case where the timer-based packet datadrop and AQM-based packet data drop are used together;

FIG. 9B depicts an example of a case where the timer-based packet datadrop and the AQM-based packet data drop are used together (at thebeginning of drop);

FIG. 9C depicts an example of a case where the timer-based packet datadrop and the AQM-based packet data drop are used together (time 30 haselapsed);

FIG. 10 depicts an example of the configuration of the units in the userequipment (UL);

FIG. 11 depicts an example of the configuration of the units in the userequipment (DL);

FIG. 12 depicts an example of the configuration of the units in the basestation (UL);

FIG. 13 depicts an example of the configuration of the units in the basestation (DL);

FIG. 14A depicts outlining operations of a first described embodiment;

FIG. 14B depicts outlining operations of the first embodiment (at thebeginning of drop);

FIG. 14C depicts outlining operations of the first embodiment (time 30has elapsed);

FIG. 15 depicts a flowchart of packet data processing at the userequipment (UL) or the base station (DL) according to the firstembodiment;

FIG. 16 depicts a flowchart of packet data processing at the userequipment (DL) or the base station (UL) according to the firstembodiment;

FIG. 17A depicts outlining operations of the user equipment beforehandover (HO) according to a second described embodiment;

FIG. 17B depicts outlining operations of the source base station beforeHO according to the second embodiment;

FIG. 18A depicts outlining operations of the user equipment after HOaccording to the second embodiment;

FIG. 18B depicts outlining operations of the target base station afterHO according to the second embodiment;

FIG. 19 depicts a flowchart of packet data processing at the userequipment (UL) according to the second embodiment (AQM is applied to anun-transmitted PDCP SDU);

FIG. 20 depicts a flowchart of packet data processing at the source basestation (UL) according to the second embodiment;

FIG. 21 depicts a flowchart of packet data processing at the target basestation (UL) according to the second embodiment;

FIG. 22A depicts outlining operations of the user equipment before HOaccording to a third described embodiment;

FIG. 22B depicts outlining operations of the source base station beforeHO according to the third embodiment;

FIG. 23A depicts outlining operations of the user equipment after HOaccording to the third embodiment;

FIG. 23B depicts outlining operations of the target base station afterHO according to the third embodiment;

FIG. 24 depicts a flowchart of packet data processing at the userequipment (UL) according to the third embodiment;

FIG. 25A depicts operations of the source base station before HOaccording to a fourth described embodiment;

FIG. 25B depicts operations of the user equipment before HO according tothe fourth embodiment;

FIG. 26A depicts an example of operations of the source base stationafter HO according to the fourth embodiment;

FIG. 26B depicts an example of operations of the target base stationafter HO according to the fourth embodiment;

FIG. 26C depicts an example of operations of the user equipment after HOaccording to the fourth embodiment;

FIG. 27A depicts another example of operations of the source basestation after HO according to the fourth embodiment;

FIG. 27B depicts another example of operations of the target basestation after HO according to the fourth embodiment;

FIG. 27C depicts another example of operations of the user equipmentafter HO according to the fourth embodiment;

FIG. 28 depicts a flowchart of packet data processing at the source basestation (DL) according to the fourth embodiment;

FIG. 29 depicts a flowchart of packet data processing at the target basestation (DL) according to the fourth embodiment;

FIG. 30 depicts a flowchart of packet data processing at the userequipment (DL) according to the fourth embodiment;

FIG. 31A depicts operations of the source base station before a delaytime added according to a fifth described embodiment;

FIG. 31B depicts operations of the source base station after a delaytime added according to the fifth embodiment;

FIG. 31C depicts operations of the target base station according to thefifth embodiment;

FIG. 31D depicts operations of the user equipment according to the fifthembodiment;

FIG. 32 depicts a GTP header structure;

FIG. 33 depicts a time stamp;

FIG. 34 depicts an example of a case where a time stamp is sent on aSequence Number (SN) State Transfer message;

FIG. 35 depicts another example of a case where a time stamp is sent onthe SN State Transfer message;

FIG. 36 depicts a flowchart of packet data processing at the source basestation (DL) according to the fifth embodiment;

FIG. 37 depicts a flowchart of packet data processing at the target basestation (DL) according to the fifth embodiment;

FIG. 38A depicts operations of the source base station according to asixth described embodiment;

FIG. 38B depicts operations of the target base station before a delaytime added according to the sixth embodiment;

FIG. 38C depicts operations of the target base station after a delaytime added according to the sixth embodiment; and

FIG. 38D depicts operations of the user equipment according to the sixthembodiment.

BRIEF DESCRIPTION OF EMBODIMENTS

Embodiments of the present invention are described below with referenceto the figures.

(1) Communication System Configuration

FIG. 1 is a diagram illustrating an example of a mobile communicationsystem configuration according to an embodiment of the presentinvention.

In FIG. 1, reference numeral 100 designates a user equipment, 200 and300 designate base stations, 400 and 500 designate cells (communicationareas) covered by the base station 200 and the base station 300,respectively, and 600 designates an upper station.

This embodiment is described below as an example of a case where theuser equipment 100 is moving from the communication area of the basestation 200, that communicated with the user equipment, to thecommunication area of the base station 300.

The base station 200 will be referred to as a source base station, andthe base station 300 will be referred to as a target base station.

(2) PDCP

FIG. 2 is a diagram illustrating a user plane protocol stack between theuser equipment and the base station according to the 3GPP specification.

In FIG. 2, PHY designates a Physical Layer, MAC designates a MediumAccess Control (MAC) Layer, RLC designates an Radio Link Control (RLC))Layer, and PDCP designates a Packet Data Convergence Protocol (PDCP)Layer.

The MAC Layer and the Physical Layer perform radio transmission byapplying modulation/demodulation techniques appropriate for the radiopropagation condition to packet data received from the RLC Layer. TheRLC Layer is a layer for controlling radio transmission. The RLC Layerperforms disassembly/assembly appropriate for the radio transmissioncondition on the packet data received from the PDCP Layer and sequencecontrol on the packet data received from the Physical Layer. The PDCPLayer performs processing including header compression and encryption onthe data received from an upper layer.

(3) Handover

FIG. 3 is a diagram outlining packet data transfer performed inaccordance with a handover operation when the user equipment is movingbetween the base stations.

In FIG. 3, the same components as those illustrated in FIG. 1 aredesignated by the same reference numerals.

While the user equipment 100 is moving from the communication area 400to the communication area 500, the user equipment 100 performs handoverto switch the base station to communicate with from the base station 200to the base station 300.

In the handover, the packet data that has been received by the basestation 200 before the handover is transferred from the base station 200to the base station 300. If the base station 200 receives data destinedto the user equipment 100 from the upper station before the transferprocess ends, that received data may be directly transferred to the basestation 300.

In FIG. 3, n, n-1, and n-2 are PDCP SDU Sequence Numbers (PDCP SDU SNs).The packet data is transferred by the unit of PDCP SDU. Although thefigure illustrates a case where a wired link directly connects the basestation 200 and the base station 300, the base stations may be logicallylinked over the upper station instead of directly connected by a wiredlink.

FIG. 7 is depicts an exemplary sequence of the handover illustrated inFIG. 3.

In FIG. 7, UE designates the user equipment which corresponds to theuser equipment 100, BS1 designates the source base station whichcorresponds to the base station 200, BS2 designates the target basestation which corresponds to the base station 300, and GW designates theupper station which corresponds to the upper station 600.

The user equipment 100 sends a Measurement Report signal that containsinformation on the measurement of the reception level of a signalreceived from the target base station 300 to the source base station 200(“1. Measurement Control”).

The source base station 200 determines whether or not to perform ahandover to the target base station 300 whose signal reception level isthe highest at the user equipment based on the information included inthe received Measurement Report signal (“2. handover (HO) TargetDetermination”).

The source base station 200 sends a Handover Request signal to thetarget base station 300 (“3. HO Request”). The source base station 200also sends information on the user equipment 100, which may include, forexample, the user equipment ID and Quality of Service (QoS) with theHandover Request signal. The target base station 300 performs a callaccept control based on the received information (4. “Call AcceptControl”).

When the target base station 300 approves the communication with theuser equipment 100, the target base station 300 sends a Handover Replysignal to the source base station 200 (“5. HO Reply”).

The source base station 200 sends a Handover Instruction signal to theuser equipment 100 (“6. HO Instruction”) and sends information on thestate of the PDCP SN to the target base station 300 (“7. SN StateTransfer”) and then starts to transfer packet data to the target basestation 300.

When the user equipment 100 receives the Handover Instruction signal,the user equipment establishes synchronization with the target basestation 300 by L1/L2 signaling (“8. L1/L2 signaling”). When thesynchronization is established, the user equipment 100 sends a HandoverCompletion signal that indicates the completion of the handover to thetarget base station 300 (“9. HO Completion”).

When the target base station 300 receives the Handover Completion signalfrom the user equipment 100, the target base station 300 sends theHandover Completion signal that indicates the completion of the handoverto the upper station 600 (“10. HO Completion”).

When the upper station 600 receives the Handover Completion signal, theupper station 600 switches the packet data transmission channel from thesource base station 200 to the user equipment 100 to the channel fromthe target base station 300 to the user equipment 100 (“11. ChannelSwitch”). Then, the upper station 600 sends a Handover Completion Replysignal to the target base station 300 (“12. HO Completion Reply”).

When the target base station 300 receives the Handover Completion Replysignal from the upper station 600, the target base station 300 sends aResource Release signal that indicates the completion of the handover tothe source base station 200 (“13. Resource Release”).

When the source base station 200 receives the Resource Release signal,the radio resource between the source base station 200 and the userequipment 100 is released (“14. Resource Release”).

FIG. 4 is an exemplary flowchart illustrating operations of the units inthe user equipment upon handover.

At S1, the user equipment sends the measurement of the signal receptionlevel to the source base station.

At S2, the user equipment receives the Handover Instruction signal sentfrom the source base station.

After receiving the Handover Instruction signal at S2, the userequipment establishes synchronization with the target base station byL1/L2 signaling at S3.

After confirming that synchronization with the target base station isestablished at S3, the user equipment reports the completion ofsynchronization, that is, completion of the handover, to the target basestation at S4.

FIG. 5 is an exemplary flowchart illustrating operations of the units inthe source base station upon handover.

At S5, the source base station receives Measurement Report signal,information on the measurement of the reception level of the signal thatis received by the user equipment from the base station.

At S6, the source base station determines whether to perform a handoverto the target base station based on the information included in theMeasurement Report signal received at S5. If the source base stationdetermines to perform the handover, the operation proceeds to S7. If thesource base station determines not to perform the handover, theoperation returns to S5.

After determining to perform the handover at S6, the source base stationsends the Handover Request signal to the target base station at S7.

At S8, the source base station receives the Handover Reply signal thatis sent from the target base station.

After receiving the Handover Reply signal at S8, the source base stationsends the Handover Instruction signal to the user equipment at S9.

At S10, the source base station sends information on the state of thePDCP SN to the target base station. The source base station transfersthe SN that is to be given next by the target base station or reportsthe SN of the PDCP SDU that has not been sent from the source basestation, for example.

At S11, the source base station receives the Resource Release signalfrom the target base station, and releases the communication connectionwith the user equipment.

FIG. 6 is an exemplary flowchart illustrating operations of the units inthe target base station upon handover.

At S12, the target base station receives the Handover Request signalsent from the source base station.

At S13, the target base station determines whether to approve the callacceptance of the communication with the user equipment. If the targetbase station approves the call acceptance, the operation proceeds toS14. If the target base station does not approve the call acceptance,the operation jumps to S19.

At S14, the target base station sends the Handover Reply signal to thesource base station.

At S15, the target base station establishes synchronization with theuser equipment by L1/L2 signaling.

After establishing synchronization with the user equipment at S15, thetarget base station receives the Handover Completion signal sent fromthe user equipment at S16.

After receiving the Handover Completion signal at S16, the target basestation sends the Handover Completion signal that indicates thecompletion of the handover to the upper station at S17.

At S18, the target base station receives the Handover Completion Replysignal sent from the upper station.

After receiving the Handover Completion Reply signal at S18, the targetbase station sends the Resource Release signal to the source basestation at S19.

(4) Packet Data Drop (a) PDCP Data Drop

One of the ways to speed up the transfer-rate of mobile communication isto reduce the transmission delay. This is achieved by means of a PDCPSDU Drop mechanism implemented at the PDCP layer of each of the userequipment and the base station.

At the PDCP layer, data sent out from an upper layer is made into packetdata as a PDCP SDU.

The packet data is dropped by the unit of PDCP SDU in a timer-baseddropping method or an AQM-based dropping method.

(b) Timer-Based Packet Data Dropping Method

FIGS. 8A & 8B are a diagrams illustrating an example of outlinedoperations of a timer-based packet data dropping method.

In FIGS. 8A & 8B, reference numeral 700 designates a PDCP SDU buffer, amemory in the radio communication device, the user equipment or the basestation. In the timer-based PDCP SDU dropping method, a residing timefor each PDCP SDU is counted down, and the PDCP SDU buffer drops thePDCP SDU whose timer is expired.

The digits 1 to 10 stored in the PDCP SDU buffer 700 indicates PDCPSDUs. Hereinafter, the PDCP SDUs are referred to as packet data. 1 to 3are packet data that have been subjected to processing including headercompression and data encryption. 1 to 3 have the SNs given and forwardedto lower layers. 4 to 10 are packet data that do not have SNs yet.Hereinafter, SNs in that state will be enclosed by square brackets [ ]in the drawings.

Packet data yet to be given the SN may be undergoing processingincluding header encryption or data encryption. The packet data yet tobe given the SN may be data received from an upper station andtemporarily stored without the SN given, or data just received from anupper station. Although those kinds of packet data yet to be given theSN may be kept in a memory other than the PDCP SDU buffer, it is assumedthat the packet data is unconditionally stored in the PDCP SDU buffer inthe description below.

“Drop Time” is the upper limit, which is given a value, set to a DropTimer (hereinafter, referred to as drop timer) that covers the entirePDCP layer. Drop Time indicates a time left for the PDCP SDU buffer todrop the packet data. In this embodiment, the upper limit of the droptimer is 100.

The value may be individually set to the drop timer according to theapplication or radio bearer. In the case of a Guaranteed Bit Rate (GBR)application, the drop timer values may be individually set to servicessuch as Realtime, Gaming, VoIMS, Streaming and the like. In the case ofa non GBR application, the drop timer values may also be individuallyset to services such as IMS Signaling, Interactive Gaming, TCPInteractive, Preferred TCP bulk data, Best Effort TCP Data and so on.LTE labels the packet data available for those applications a QoS ClassIdentifier (QCI) so that the base station determines the applicationtype based on the QCI.

The upper limit of the drop timer is variable. The value is reported tothe target base station at “3. HO Request”, “7. SN State Transfer”, and“9. HO Completion” in FIG. 7, for example.

In the case where the upper limit of the drop timer is reported from thesource base station at “3. HO Request”, the target base station candetermine whether to accept the request by taking into account the droptimer value.

In the case where the upper limit of the drop timer is reported from thesource base station at “7. SN State Transfer”, the target base stationreceives the upper limit after accepting the call control. That canreduce signaling overhead.

In the case where the upper limit of the drop timer is reported to thetarget base station at “9. HO Completion”, the upper limit is reportedby signaling from the upper station. In this case, unlike theabovementioned two cases, the upper station performs signaling only tothe target base station if the source base station and the target basestation are not directly linked. That can reduce signaling overhead.

The “drop timer” indicates the drop timer value for the packet data thatis stored in the PDCP SDU buffer 700, i.e., a time elapsed since thedrop timer started.

FIG. 8A and FIG. 8B are diagrams illustrating a case in which the packetdata 1 to 8 is stored in the PDCP SDU buffer 700.

In the PDCP SDU buffer 700 (FIG. 8A), the drop timers for the packetdata 1 to 8 indicate 80, 70, . . . , 20, and 10, respectively. That is,the drop timers for all the packet data have not exceeded the upperlimit 100. In other words, as the drop timers have not expired, thepacket data is still stored in the PDCP SDU buffer 700.

In the PDCP SDU buffer 700 after the time 30 elapsed (FIG. 8B), the droptimers for the packet data 1 to 10 indicate 110, 100, . . . , 50, and40, respectively. The drop timer values for the newly transferred packetdata 9 and 10 indicate 0.

The drop timers for the packet data 1 and 2 indicate 110 and 100,reaching and exceeding the upper limit thereof, respectively. As thedrop timer has expired, the packet data is dropped.

As mentioned above, this embodiment is adapted to set the upper limitfor a time the PDCP SDU buffer 700 is allowed to store the packet dataand drop the packet data that has exceeded the upper limit. That enablesthe data transmission to meet the minimum guaranteed delay for theapplication even under a poor radio condition.

(c) AQM-Based Packet Data Dropping Method

Active Queue Management (AQM) is a buffer managing technique used in arouter, or a drop mechanism accompanying a buffer, to enable efficientcongestion control of Transport Control Protocol (TCP) to providereliable communication mainly on an end-to-end basis. The AQM techniquedeliberately drops a TCP segment before a buffer overflowing eventforces the TCP segment to be dropped at a node on a network. In thismanner, the AQM technique prevents packet data from being sequentiallydropped to avoid TCP time out. That means the AQM technique can adjustthe throughput of TCP before the network enters a serial congestionstate, i.e., while the network is in a light congestion state.Accordingly, the AQM technique can provide congestion control whileefficiently using the network resource and maintaining high TCPthroughput.

FIG. 9A, FIG. 9B and FIG. 9C are diagrams outlining examples of a packetdata drop operation method in a case where the timer-based packet datadrop and the AQM-based packet data drop are used together.

In FIG. 9A, FIG. 9B and FIG. 9C, reference numeral 701 designates apacket drop control unit that controls over packet drop. The samecomponents as those illustrated in FIG. 8A or FIG. 8B are designated bythe same reference numerals.

In the PDCP SDU buffer 700 in FIG. 9A, the drop timers for the packetdata 1 to 8 indicate 80, 70, . . . , 20, and 10, respectively. That is,as the drop timers (upper limit 100) for all the packet data have notexpired, the packet data is still stored in the PDCP SDU buffer 700.

When the AQM-based packet data drop starts, as illustrated in FIG. 9B,the packet drop control unit 701 controls selection and drop of packetdata. The packet data to drop may be selected at random.

When the time 30 elapsed for example, as illustrated in FIG. 9C, thepacket data whose drop timer has expired is dropped in the same manneras that described in FIG. 8B. FIG. 9C indicates that the packet data 1is being dropped.

(5) Configurations and Operations of the Units Used for the Embodiment(a) User Equipment

FIG. 10 is a diagram illustrating an example of the configuration of theunits in the user equipment and the units' configuration for Uplink (UL)communication.

In FIG. 10, reference numeral 101 designates a buffer unit, 102designates a sending/receiving unit, 103 designates an antenna, 104designates a control unit, 105 designates a measurement unit, and 106designates a timer managing unit.

The buffer unit 101 represents a buffer for the MAC layer, a buffer forthe RLC layer, and a buffer for the PDCP layer as a single buffer;though, the buffers may be arranged in a physically separated design.

In the buffer for the PDCP layer, data received from an upper layer ismade into packet data as a PDCP SDU, an SN is given to the packet data,and header compression and encryption are performed on the packet data.

In the buffer for the RLC layer, disassembly/assembly is performed onthe data forwarded from the PDCP layer.

In the buffer for the MAC layer, modulation/demodulation techniquesappropriate for the radio propagation condition are performed.

The sending/receiving unit 102 performs error correction and modulationsuch as encoding on the data forwarded from the buffer, and sends outthe processed data to the antenna 103.

Also, the sending/receiving unit 102 receives various types of controlinformation for performing UL data transmission. Here, the controlinformation includes but is not limited to available radio resourcesinformation, such as frequency information, time information, and so on,and transmission power information.

The control unit 104 performs various types of control in the userequipment such as scheduling request for UL data, reporting the basestation on the amount of data arrived at buffers, reporting on ChannelQuality Information (CQI), sending the Sounding Reference Signal (RS),and designation of an intermittent transmission mode.

Also, the control unit 104 controls the AQM-based packet data drop.

The measurement unit 105 measures radio qualities from the base stationsin preparation for the handover. The measurement unit 105 may alsomeasure the signal intensity and the quality of a pilot signal, and thesignal intensity for a carrier frequency.

The timer managing unit 106 manages the drop timer for the packet data.

As various timers are used in lower layers such as the RLC layer, thetimer managing unit 106 may also control the timers. The timer managingunit 106 manages the upper limit time for resending packet data in sucha case where the packet data forwarded from the PDCP layer isdisassembled/assembled as required at the RLC layer and treated as thepacket data in the RLC layer.

The measurement unit 105 or the timer managing unit 106 may be includedin or provided separately from the control unit 104.

FIG. 11 is a diagram illustrating an example of the configuration of theuser equipment units and the configuration of units for Downlink (DL)communication.

In FIG. 11, reference numeral 107 designates the antenna, 108 designatesthe sending/receiving unit, 109 designates the buffer unit, 110designates a reorder unit, 111 designates the control unit, or thecontrol unit, 112 designates the measurement unit, 113 designates abuffer managing unit, 114 designates a reorder managing unit, and 115designates the timer managing unit.

The sending/receiving unit 108 performs demodulation on the data that issent from the base station and received by the antenna 107, and sendsthe demodulated data to the buffer unit 109. Also, the sending/receivingunit 108 receives various types of control information for performing DLdata transmission. Here, the control information includes, for example,radio resource information used for the data transmission.

The buffer unit 109 temporarily stores the data sent from thesending/receiving unit 108 in order to forward the data to an upperlayer.

In this embodiment, a buffer for the MAC layer, a buffer for the RLClayer, and a buffer for the PDCP layer are represented as a singlebuffer; though, the buffers may be arranged in a physically separateddesign.

At the MAC layer, bit error detection is performed on the data forwardedfrom the sending/receiving unit. If a bit error is detected, NACK isreturned to the base station and resending is performed. If no bit erroris detected, ACK is returned to the base station and the data isforwarded to the RLC layer.

At the RLC layer, whether the received data are in the correct order ornot is determined. If the data are not in the correct order, data yet tobe received is waited for. If the data is in the correct order, the datais forwarded to the PDCP layer.

At the PDCP layer, decryption and header expansion are performed on thereceived data and the PDCP SDU is reconstructed.

The reorder unit 110 sorts the data, or packet data, sent from thebuffer unit 109 in the order they are sent, or in the order of PDCP SNs,and forwards the data to an upper layer. If a PDCP SN is lost, that isif there are packet data lost in a transmission failure, a PDCP SN yetto be received is waited for and forwarded.

The control unit 111 performs various types of control in the userequipment such as scheduling request for the UL data, reporting the basestation on the amount of data arrived at buffers, reporting on CQI,sending the Sounding RS, and designation of the intermittenttransmission mode.

Also, the control unit 111 controls the AQM-based packet data drop.

The measurement unit 112 measures radio qualities for signals receivedfrom the base stations in preparation for the handover. The measurementunit 112 may also measure the signal intensity and the quality of apilot signal, and the signal intensity for a carrier frequency.

The buffer managing unit 113 controls the data stored in the buffer unit109.

The buffer managing unit 113 performs bit error detection/resendingcontrol on the data at the MAC layer, order control/resending control onthe data at the RLC layer, and decryption/header expansion control onthe data at the PDCP layer, for example.

The buffer managing unit 113 forwards the data of a reconstructed PDCPSDU to the reorder unit 110.

When a bit error is detected by the buffer managing unit 113, data is tobe resent. For that purpose, Hybrid Automatic Repeat Request (HARQ) isapplied at the MAC layer. If the data block for which the bit error isdetected at the MAC layer cannot be recovered, all of the data blocksincluding the erroneous data block will be resent, or forwarded to theMAC layer again, until a given time clocked by the timer at the RLClayer.

HARQ is an ARQ technique using a combination of Automatic Repeat Requestand Forward Error Correction (FEC). Specifically, error correctionencoding is performed on the data blocks of the information bit byadding a parity bit for error detection to the data block at the sendingside, and all or a part of the data blocks are sent. If resending isrequired, all or a part of the encoded bit corresponding to the currentdata block is resent. At receiving side, data is composed of each bitcorresponding to each existing data block among the resent data blocks.Then, by using the composed blocks, the error correction and the errordetection are performed again. At the receiving side, a decoding trialperformed by replying ACK/NACK and resending data to the sending side isrepeated in the described manner until no erroneous data block is leftwithin the number of times of a given upper limit.

The reorder managing unit 114 manages reordering of the packet data. Thereorder managing unit 114 determines packet data lost in a transferfailure, and also detects packet data whose drop timer has expired.Then, the reorder managing unit 114 drops that packet data.

The timer managing unit 115 manages the drop timer for the packet dataand reordering timer which is the timer for reordering the packet data.

The value set to the reordering timer at the timer managing unit 115 maybe the reordering timer value that is decided at the initialization ofvarious parameters for the PDCP. The value selected by the base stationfrom a set of given reordering timer values may also be used. The basestation may select the value by taking account of an average receptiondelay due to the DL radio condition, a delay time for received packetdata, the QCI that accompanies the application or radio bearer.

The measurement unit 112, the buffer managing unit 113, the reordermanaging unit 114, and the timer managing unit 115 may be included in orprovided separately from the control unit 111.

The user equipment includes all or a part of the units illustrated inFIG. 10 and FIG. 11; though, the user equipment may have the antenna 103and the antenna 107, the control unit 104 and the control unit 111, themeasurement unit 105 and the measurement unit 112, and the timermanaging unit 106 and the timer managing unit 115 in common.

(b) Base Station

FIG. 12 is a diagram illustrating an example of the configuration of thebase station units and operations of the units for the UL communication.

In FIG. 12, reference numeral 201 designates the antenna, 202 designatesthe sending/receiving unit, 203 designates the buffer unit, 204designates the reorder unit, 205 designates the control unit, 206designates the measurement unit, 207 designates the buffer managingunit, 208 designates an handover determination unit, and 209 designatesthe timer managing unit.

The sending/receiving unit 202 performs demodulation on the data that issent from the user equipment and received by the antenna 201, and sendsthe demodulated data to the buffer unit 203. Also, the sending/receivingunit 202 receives various types of control information for performingthe UL data transmission. Here, the control information includesSounding RS, for example.

The buffer unit 203 temporarily stores the packet data sent from thesending/receiving unit 202 to forward the data to an upper layer.

The reorder unit 204 sorts the data, or packet data, sent from thebuffer unit 203 in the order they are sent, or in the order of PDCP SNs,and forwards the data to an upper layer. In LTE, the reorder unit 204transmits data received in the correct order to an upper layer. If thereorder unit 204 detects data yet to be received, the reorder unit 204transfers data thereafter to the target base station.

The control unit 205 performs various types of control in the basestation such as scheduling approval for the UL data, resource allocationaccording to the amount of data, the Quality of Service (QoS) and thelike, and designation of the intermittent receiving mode.

The measurement unit 206 measures the DL radio quality based on the CQIreported from the user equipment and the UL radio quality based on theSounding RS.

Also, the control unit 205 controls the AQM-based packet data drop.

The buffer managing unit 207 controls the data stored in the buffer unit203.

The buffer managing unit 207 performs bit error detection/resendingcontrol on the data at the MAC layer, order control/resending control onthe data at the RLC layer, and decryption/header expansion control onthe data at the PDCP layer, for example. The buffer managing unit 207forwards the data of a reconstructed PDCP SDU to the reorder unit 204.

The HO determination unit 208 determines whether to perform the handoverbased on the measurement of the radio signal quality from themeasurement unit 206.

The timer managing unit 209 manages the drop timer for the packet dataand the reordering timer for reordering the packet data.

The value set to the reordering timer at the timer managing unit 209 isset in the same manner as that taken for the reordering timer at theuser equipment.

The reorder managing unit 210 manages reordering of the packet data. Thereorder managing unit 210 determines packet data lost in a transferfailure, and also detects packet data whose drop timer has expired.Then, the reorder managing unit 210 drops that packet data.

The measurement unit 206, the buffer unit 207, the HO determination unit208, the timer managing unit 209, and the reorder managing unit 210 maybe included in or provided separately from the control unit 205.

FIG. 13 is a diagram illustrating an example of the configuration of theunits in the base station and operations of the units for the DLcommunication.

In FIG. 13, reference numeral 211 designates the antenna, 212 designatesthe buffer unit, 213 designates a scheduler unit, 214 designates thesending/receiving unit, 215 designates the control unit, 216 designatesa delay measuring unit, 217 designates the timer managing unit, 218designates a scheduler managing unit, 219 designates the measurementunit, and 220 designates the HO determination unit.

The buffer unit 212 represents a buffer for the MAC layer, a buffer forthe RLC layer, and a buffer for the PDCP layer as a single buffer inthis embodiment; though, the buffers may be arranged in a physicallyseparated design.

In the buffer for the PDCP layer, data received from an upper layer ismade into packet data as a PDCP SDU, an SN is given to the packet data,and header compression and encryption are performed on the packet data.

In the buffer for the RLC layer, disassembly/assembly is performed onthe data forwarded from the PDCP layer. In the buffer for the MAC layer,modulation/demodulation techniques appropriate for the radio propagationcondition are performed on the data forwarded from the RLC layer.

When the handover is carried out, the data forwarded to the RLC layerand the MAC layer can be transmitted until the radio link isdisconnected. The data forwarded after the disconnection is dropped.

The source base station transfers data at the PDCP layer to the targetbase station by the unit of PDCP SDU. On the other hand, the upperstation can transfer the packet data to the target base station withoutgiving the PDCP SN.

If pieces of data destined to the user equipment are stored in thebuffer unit 212, the scheduler unit 213 selects a piece of data to sendfrom the stored pieces of data and sends out the selected piece of datato the sending/receiving unit 214.

The sending/receiving unit 214 performs the error correction andmodulation such as encoding of the data sent from the scheduler unit213, and sends out the processed data to the antenna 211. Also, thesending/receiving unit 214 receives various types of control informationfor performing the DL data transmission. Here, the control informationincludes radio resource information, for example.

The control unit 215 performs various types of control in the basestation such as scheduling approval for the UL data, resource allocationaccording to the amount of data, the QoS and so on, designation of theintermittent receiving mode, and designation of HO. The control unit 215also controls the AQM-based packet data drop.

The delay measuring unit 216 measures/estimates a delay time, forinstance, that occur upon transmission of the data to the userequipment, the upper station, another base station and so on.

The timer managing unit 217 manages the drop timer.

As various timers are used in lower layers such as the RLC layer, thetimer managing unit 217 may also control the timers. The timer managingunit 217 manages the limit for resending packet data in such a casewhere the packet data forwarded from the PDCP layer isdisassembled/assembled as required at the RLC layer and treated as thepacket data in the RLC layer.

The scheduler managing unit 218 manages the scheduler unit 213 bysending out a signal that indicates the user equipment to be authorizedto transmit to the scheduler unit 213 based on the control informationfrom the user equipment received by the sending/receiving unit 214.

The measurement unit 219 measures the DL radio quality based on the CQIreported from the user equipment and the UL radio quality based on theSounding RS.

The HO determination unit 220 determines whether to perform the handoverbased on the measurement of the radio quality from the measurement unit219.

The base station includes all or a part of the units illustrated in FIG.12 and FIG. 13; though, the base station may have the antenna 201 andthe antenna 211, the control unit 205 and the control unit 215, themeasurement unit 206 and the measurement unit 219, the HO determinationunit 208 and the HO determination unit 220, and the timer managing unit209 and the timer managing unit 217 in common.

(6) First Embodiment (a) Overview

According to the first embodiment, the PDCP SDU, hereinafter, referredto as packet data, stored in a PDCP SDU buffer, hereinafter, referred toas buffer unit, is dropped by the same operations as those shown in FIG.9B in the radio communication device, the user equipment or the basestation, but the packet data forwarded to a lower layer is excluded fromthe packet data drop candidates.

FIG. 14A, FIG. 14B and FIG. 14C are diagrams outlining operations of afirst embodiment according to the present invention.

In FIG. 14A, FIG. 14B and FIG. 14C, the same components as thoseillustrated in FIG. 9A and FIG. 9B are designated by the same referencenumerals.

Among the packet data that are stored in the buffer unit, the packetdata 1 to 3 that have been forwarded to a lower layer are excluded fromthe packet data drop candidates before the drop timer is expired.

That is, when the AQM-based packet data drop is applied to the packetdata whose drop timers have not expired, the packet data to be droppedis selected from the packet data 4 to 8.

At the stage illustrated in FIG. 14B in which all the packet data hasthe drop timers unexpired, the packet data 4 and 7 is dropped by theAQM-based packet data dropping method. At the later stage illustrated inFIG. 14C, the packet data 1 and 2 whose drop timers have expired aredropped.

(b) Processing Flow

FIG. 15 is a flowchart of packet data processing at the user equipmentfor UL or packet data processing at the base station for DL according tothe first embodiment, i.e., where the user equipment or the base stationoperates as a transmitter.

At S20, the buffer unit in the user equipment or the base stationreceives data sent from an upper layer.

At S21, the user equipment or the base station makes the data receivedby the buffer unit at S20 into packet data as a PDCP SDU (PDCP SDUconstruction).

At S22, the user equipment or the base station starts the drop timer foreach packet data constructed at S21.

At S23, the user equipment or the base station determines whether thedrop timer that is started at S22 have expired for each packet data. Ifthe drop timer has expired, the operation proceeds to S24. If the droptimer has not expired, the operation proceeds to S25.

If the drop timer has expired at S23, the packet data that has beenforwarded to a lower layer is dropped at S24.

If the drop timer has not expired at S23, the user equipment or the basestation selects the packet data that has not been forwarded to a lowerlayer from the packet data that is stored in the buffer unit at S25.

At S26, the user equipment or the base station determines whether theAQM-based packet data drop is to be applied to the packet data selectedat S25. If the AQM-based packet data drop is to be applied, theoperation proceeds to S27. Otherwise, the operation jumps to S29.

If it is determined that the AQM-based packet data drop is to be appliedat S26, the user equipment or the base station selects the packet datato drop at S27.

At S28, the user equipment or the base station drops the packet dataselected at S27.

At S29, the buffer unit in the user equipment or the base stationforwards the packet data to a lower layer.

FIG. 16 is a flowchart of packet data processing at the user equipmentfor DL or packet data processing at the base station for UL according tothe first embodiment, i.e., where the user equipment or the base stationoperates as a receiver.

At S30, the buffer unit at the MAC layer of the user equipment or thebase station receives the radio transmitted data.

At S31, the user equipment or the base station performs bit errordetection on the received data. If an error is detected, the operationproceeds to S32. Otherwise, the operation proceeds to S34.

At S32, the user equipment or the base station waits for reception ofthe data yet to be received at the RLC layer with a timer counting down.If the timer has not expired, the operation returns to S30. Otherwise,the operation proceeds to S33.

As the resending timer at the RLC layer has expired, the user equipmentor the base station drops the data concerned at S32.

At S34, the user equipment or the base station reconstructs the PDCP SDUwith the data from which no error has been detected.

At S35, the user equipment or the base station transmits the PDCP SDUsto the upper station in the order they were received.

The user equipment or the base station may perform reordering here andtransmit the PDCP SDUs to the upper station in the order of SNs.

(7) Second Embodiment (a) Overview

In the second embodiment, an example of packet drop operations in thecase where a part of data, or packet data, is not successfully sent inthe UL communication from the user equipment to the source base stationdue to a radio error before the handover is described.

FIG. 17A, FIG. 17B, and FIG. 18A, FIG. 18B are diagrams outliningoperations of the second embodiment before the handover and after thehandover, respectively.

In FIG. 18A, reference numeral 702 designates the packet drop controlunit that is assumed to be included in the control unit 104 illustratedin FIG. 10.

FIG. 17A and FIG. 17B illustrate a case before the handover where, amongthe packet data stored in the buffer unit 101 in the user equipment, thepacket data 1 and the packet data 3 have been sent from the userequipment to the source base station but the packet data 2 has not sentuntil the radio link is switched due to a bit error. In the buffer unit203 a in the source base station, the packet data 1 and the packet data3 are stored.

Upon the handover, as the data up to the packet data 1 has been receivedin the order of SNs, the source base station transmits the data to theupper station. As the packet data 2 has not been received yet, thepacket data 3 is transferred to the target base station.

FIG. 18A and FIG. 18B illustrate a case after the handover, where theuser equipment sends the packet data to the target base station. Here,the user equipment sends or resends the packet data in the order of SNsstarting from the packet data 2 that the user equipment failed to sendbefore the handover.

In this embodiment, once the handover is performed, the drop timervalues are reset for the packet data that has been forwarded to a lowerlayer and needs to be resent to the target base station, among thepacket data stored in the buffer unit 101 in the user equipment.Specifically, the user equipment controls to make the drop timer values0 for the packet data 2 and the packet data 3.

As the reordering timer values are appropriately set at the PDCP layerof the target base station, the user equipment can determine that thePDCP SDU that has not been received yet when the timer expired is losteven if the user equipment has reset the drop timer values. Accordingly,the user equipment need not wait for the arrival of the lost PDCP SDU.The user equipment can forward the PDCP SDUs thereafter to the upperstation in order. Thus, the application quality is not affected in thiscase.

If the packet data 2 and the packet data 3 have been forwarded to alower layer, the user equipment can exclude the packet data 2 and thepacket data 3 from the AQM-based packet data drop candidates. Otherwise,the user equipment can include them in the packet data drop candidates.The user equipment can also exclude only the packet data 2 thatunderwent a transmission error from the AQM-based packet data dropcandidates.

The target base station stores the packet data received from the userequipment or via the source base station in the buffer unit 203 b, andperforms reordering to sort the already received packet data and newlyreceived packet data in the order of SNs.

(b) Processing Flow

FIG. 19 is a flowchart of packet data processing in the UL communicationat the user equipment.

At S36, the buffer unit in the user equipment receives data sent from anupper layer.

At S37, the user equipment makes the data received by the buffer unit atS36 into packet data as a PDCP SDU (PDCP SDU construction).

At S38, the user equipment starts the drop timer for each packet dataconstructed at S37.

At S39, if the user equipment receives the Handover (HO) Instructionsignal sent from the source base station, the operation proceeds to S40.Otherwise, the operation jumps to S41.

If the user equipment receives the HO Instruction signal at S39, theuser equipment resets the drop timers that is started at S38 and freshlystarts the drop timers at S40.

At S41, the user equipment determines whether the drop timer that isstarted at S38 or S40 has expired for each packet data. If the droptimer has expired, the operation proceeds to S42. Otherwise, theoperation proceeds to S43.

If the drop timer has expired at S41, the packet data that has beenforwarded to a lower layer is dropped at S42.

If the drop timer has not expired at S41, the user equipment selects thepacket data that has not been forwarded to a lower layer from the packetdata that is stored in the buffer unit at S43.

At S44, the user equipment determines whether the AQM-based packet datadrop is to be applied to the packet data selected at S43. If theAQM-based packet data drop is to be applied, the operation proceeds toS45. Otherwise, the operation jumps to S47.

If it is determined that the AQM-based packet data drop is to be appliedat S44, the user equipment selects the packet data to drop at S45.

At S46, the packet data selected at S45 is dropped.

At S47, the buffer unit in the user equipment forwards the packet datato a lower layer.

FIG. 20 is a flowchart of packet data processing in the UL communicationat the source base station.

At S48, the buffer unit at the MAC layer of the source base stationreceives the radio transmitted data.

At S49, the source base station performs error detection on the datareceived at S48. If an error is detected, the operation proceeds to S50.Otherwise, the operation proceeds to S52.

At S50, the source base station waits for reception of the data yet tobe received at the RLC layer with a timer counting down. If the timerhas expired, the operation proceeds to S51. Otherwise, the operationreturns to S48.

At S51, the source base station drops the data whose resending timer hasexpired at S50.

At S52, the source base station makes the data from which no error hasbeen detected at S49 into packet data as a PDCP SDU (PDCP SDUconstruction).

At S53, the source base station sends the HO Instruction signal to theuser equipment.

At S54, the source base station determines whether to transfer thepacket data to the target base station. If it is to transfer the packetdata, the operation proceeds to S55. Otherwise, the operation proceedsto S60.

At S55, the source base station reports the upper limit of the droptimer to the target base station. As mentioned above, the upper limit ofthe timer is reported to the target base station at any of “3. HORequest”, “7. SN State Transfer”, and “9. HO Completion” illustrated inFIG. 7. Here, it is assumed that the upper limit is reported by the “7.SN State Transfer” message.

At S56, the source base station determines whether or not the packetdata has incurred any loss, meaning, any missing part due to atransmission failure. If the packet data has such a loss, the operationproceeds to S57. Otherwise, the operation jumps to S59.

At S57, the source base station transfers the packet data whose SNs aresmaller than that of the packet data lost to the upper station.

At S58, the source base station transfers the packet data whose SNs arebigger than that of the packet data lost to the target base station.

At S59, the upper station forwards the transferred packet data to muchupper station. The target base station forwards the packet datatransferred to a lower layer.

At S60, the source base station reports the upper limit of the droptimer to the target base station. As mentioned above, the upper limit ofthe timer is reported to the target base station at any of “3. HORequest”, “7. SN State Transfer”, and “9. HO Completion” illustrated inFIG. 7. Here, it is assumed that the upper limit is reported by the “7.SN State Transfer” message.

FIG. 21 is a flowchart of packet data processing in the UL communicationat the target base station.

At S61, the target base station receives the data from thesending/receiving unit.

At S62, the target base station determines whether the data received atS61 is the data received from the user equipment. If it is the datareceived from the user equipment, the operation proceeds to S63.Otherwise, the operation proceeds to S66.

At S63, the target base station performs error detection on the receiveddata in the buffer unit at the MAC layer. If an error is detected, theoperation proceeds to S64. Otherwise, the operation proceeds to S66.

At S64, the target base station determines whether the resending timerat the RLC layer for the data from which an error has been detected atS63 has expired. If the resending timer has expired, the operationproceeds to S65. Otherwise, the operation returns to S61.

At S65, the target base station drops the data whose resending timer hasexpired at S64.

At S66, the target base station makes the received data into packet dataas a PDCP SDU (PDCP SDU construction).

At S67, the target base station determines whether the packet data thatis constructed at S66 has incurred any loss, meaning, any missing partdue to a transmission failure. If the packet data has a loss, theoperation proceeds to S68. Otherwise, the operation proceeds to S71.

At S68, when a PDCP SDU yet to be received is detected, the target basestation starts the reordering timer.

At S69, the target base station waits for the arrival of the PDCP SDUyet to be received until the reordering timer expires.

At S70, the target base station determines whether the reordering timerhas expired. If the reordering timer has not expired, the operationreturns to S69. Otherwise, the operation proceeds to S71.

At S71, the buffer unit in the target base station forwards the packetdata to a lower layer.

(8) Third Embodiment (a) Overview

In the third embodiment, an example of packet data drop operations inthe case where a part of data, or packet data, is not successfully sentin the UL communication from the user equipment to the source basestation due to a radio error before the handover, as described in thesecond embodiment.

FIG. 22A, FIG. 22B, and FIG. 23A, FIG. 23B are diagrams outliningoperations of the third embodiment before the handover and after thehandover, respectively.

The operations of the buffer units in the user equipment, the sourcebase station, and the target base station before and after the handovershown in FIG. 22 and FIG. 23 are same as those described in FIG. 17A,FIG. 17B and FIG. 18A, FIG. 18B of the second embodiment, respectively.

The exception is that, in FIG. 23A showing the embodiment, the bufferunit 101 in the user equipment takes over the drop timer values.

For example, the drop timers for the packet data 2 and the packet data 3are 70 and 60 in FIG. 22A, respectively. The values are taken over inFIG. 23A.

(b) Processing Flow

FIG. 24 is a flowchart of packet data processing in the UL communicationat the user equipment.

At S72, the buffer unit in the user equipment receives data sent from anupper layer.

At S73, the user equipment makes the data received by the buffer unit atS72 into packet data as a PDCP SDU (PDCP SDU construction).

At S74, the user equipment starts the drop timer for each packet dataconstructed at S73.

At S75, if the user equipment receives the HO Instruction signal sentfrom the source base station, the operation proceeds to S76. Otherwise,the operation jumps to S77.

If the user equipment receives the HO Instruction signal at S75, theuser equipment maintains the drop timers that are started at S74 andtakes over the elapsed times of the packet data.

At S77, the user equipment determines whether the drop timer that isstarted at S74 has expired for each packet data. If the drop timer hasexpired, the operation proceeds to S78. Otherwise, the operationproceeds to S79.

If the drop timer has expired at S77, the user equipment drops thepacket data that has been forwarded to a lower layer at S78.

If the drop timer has not expired at S77, the user equipment selects thepacket data that has not been forwarded to a lower layer from the packetdata that is stored in the buffer unit at S79.

At S80, the user equipment determines whether the AQM-based packet datadrop is to be applied to the packet data selected at S79. If theAQM-based packet data drop is to be applied, the operation proceeds toS81. Otherwise, the operation jumps to S83.

If it is determined that the AQM-based packet data drop is to be appliedat S80, the user equipment selects the packet data to drop at S81.

At S82, the user equipment drops the packet data selected at S81.

At S83, the buffer unit in the user equipment forwards the packet datato a lower layer.

The packet data processing flows for the source base station in the ULand the target base station in the UL in this embodiment are the same asthose in the second embodiment illustrated in FIG. 20 and FIG. 21,respectively.

(9) Fourth Embodiment (a) Overview

In the fourth embodiment, an example of packet data drop operations inthe case where a part of data, or packet data, is not successfully sentin the DL communication from the source base station to the userequipment due to a radio error before the handover is described.

FIG. 25A and FIG. 25B are diagrams illustrating data transmission fromthe source base station to the user equipment before the handover.

In FIG. 25A and FIG. 25B, the packet data 1 to 3 has been processed atthe PDCP layer of the buffer unit 212 a in the source base station, andthe packet data 4 to 8 is being processed.

When the data including the packet data 1 to 3 is sent from the sourcebase station, the user equipment receives the data, makes the data intothe packet data as PDCP SDUs in the buffer unit 108 of the userequipment, and stores the packet data.

FIG. 25A and FIG. 25B illustrate a case where the packet data 1 and thepacket data 3 is successfully sent from the source base station to theuser equipment but the packet data 2 has failed in transmission beforethe radio link switch due to an error. In this embodiment, the userequipment resends, or keeps sending, the packet data 2 but does notresend the packet data 3 that is successfully sent.

FIG. 26A, FIG. 26B, and FIG. 26C and FIG. 27A, FIG. 27B, and FIG. 27Care diagrams illustrating data transfer from the source base station tothe target base station and data transmission from the target basestation to the user equipment after the handover.

In FIG. 26 and FIG. 27, reference numeral 703 designates the packet dropcontrol unit that is assumed to be included in the control unit 215illustrated in FIG. 13.

The source base station transfers the packet data 2, which the sourcebase station failed to send to the user equipment before the handover,to the target base station.

The source base station also transfers the packet data 4 to 8, which hasnot been sent before the handover, to the target base station. Here, thesource base station may complete the processing on the packet data thatis being processed in the buffer unit 212 a of the source base stationamong the packet data 4 to 8 and transfer the packet data with the SNsadded or may suspend the processing and transfer the packet data withoutthe SNs added.

The packet data 9 and 10 to be stored in the buffer unit 212 b of thetarget base station illustrated in FIGS. 26B and 27B is the packet dataincluded in the data received from the upper station.

FIG. 26A and FIG. 26B illustrate a case where the packet data 2 and thepacket data 4 to 8 are transferred from the source base station to thetarget base station at the same timing. When the buffer unit 212 b ofthe target base station receives the packet data 2 and the packet data 4to 8, the drop timers for all the transferred packet data are reset (setto 0).

FIG. 27A and FIG. 27B illustrate a case where the packet data 2 and thepacket data 4 to 8 are transferred from the source base station to thetarget base station by given intervals, every ten hours for example.When the buffer unit 212 b receives the packet data 2 and the packetdata 4 to 8, the target base station sets the drop timers with givenintervals. For example, when the target base station receives the packetdata 8, the drop timers for the packet data 2 and 4 to 8 have the valuesof 50, 40, . . . , 0, respectively.

As shown in the example illustrated in FIG. 26 and FIG. 27, the targetbase station is basically adapted to reset, or not to take over, thedrop timers; though, it may set the timers at its discretion. In one ofthe setting methods, the target base station has a fixed value set tothe timers in advance. In that case, the target base station uniformlysets 10 for the drop timer values for the transferred packet data inadvance. The target base station may also set the drop timer value foreach QCI by taking account of the value of the QCI to which each of thetransferred packets belongs. The target base station may set the droptimers also by taking account of the traffic load it bears.

In another method, the target base station estimates a transfer delayfor the transferred packet data based on delay measurement. If theestimated delay is 15, the target base station uniformly sets 15 to thetimer values.

In yet another method, the target base station estimates the transferamount of the transferred packet data and sets the timer value accordingto the estimated amount. If the estimated transfer amount of the packetdata is 10, the target base station uniformly sets 10 to the drop timervalues. If the estimated transfer amount of the packet data is 20, thetarget base station uniformly sets 20 to the drop timer values. Thetarget base station may decide the transfer amount from the transferstate of the PDCP SDU reported at “7. SN State Transfer”, instead ofestimating the transfer value. The target base station may also set thedrop timer value for each QCI by taking account of the value of the QCIto which each of the transferred packets belongs.

In the fourth embodiment, the target base station may apply theAQM-based packet data drop only to the data received from the upperstation, or may apply it to both the data received from the upperstation and the data that has not been forwarded to a lower layer amongthe data received from the source base station.

The buffer unit 108 of the user equipment reorders the packet dataincluded in the data received from the target base station and thealready stored packet data, and then forwards the packet data to anupper layer in the order the packet data sent, or in the order of SNs.In FIG. 26C and FIG. 27C, for example, the buffer unit of the userequipment forwards the packet data in the order of the packet data 2 andthe packet data 3 to an upper layer.

(b) Processing Flow

FIG. 28 is a flowchart of packet data processing in the DL communicationat the source base station.

At S84, the buffer unit in the source base station receives data sentfrom an upper layer.

At S85, the source base station makes the data received by the bufferunit at S84 into packet data as a PDCP SDU (PDCP SDU construction).

At S86, the source base station starts the drop timer for each packetdata constructed at S85.

At S87, the source base station sends the HO Instruction signal to theuser equipment.

After sending the HO Instruction signal at S87, the source base stationdetermines whether to transfer the packet data to the target basestation at S88. If the packet data is to be transferred, the operationproceeds to S89. Otherwise, the operation proceeds to S91.

If the source base station determines to transfer the packet data atS88, the source base station reports the upper limit of the drop timerfor the packet data to transfer to the target base station at S89. Asmentioned above, the upper limit of the timer is reported to the targetbase station at any of “3. HO Request”, “7. SN State Transfer” and “9.HO Completion” illustrated in FIG. 7. Here, it is assumed that the upperlimit is reported by the “7. SN State Transfer” message.

At S90, the source base station estimates a transfer delay time.

At S91, the source base station reports the upper limit of the droptimer to the target base station. As mentioned above, the upper limit ofthe timer is reported to the target base station at any of “3. HORequest”, “7. SN State Transfer”, and “9. HO Completion” illustrated inFIG. 7. Here, it is assumed that the upper limit is reported by the “7.SN State Transfer” message.

At S92, the buffer unit in the source base station forwards the packetdata to a lower layer until the radio link is disconnected.

FIG. 29 is a flowchart of packet data processing in the DL communicationat the target base station.

At S93, the buffer unit of the target base station receives the datasent from an upper layer.

At S94, the target base station makes the data received by the bufferunit at S93 into packet data as a PDCP SDU (PDCP SDU construction).

At S95, the target base station starts the drop timer for each packetdata constructed at S94.

At S96, the target base station determines whether the drop timer thatis started at S95 has expired for each packet data. If the drop timerhas expired, the operation proceeds to S97. Otherwise, the operationproceeds to S98.

If the drop timer has expired at S96, the target base station drops thepacket data that has been forwarded to a lower layer at S97.

At S98, the target base station selects the packet data received fromthe upper station.

At S99, the target base station determines whether the AQM-based packetdata drop is to be applied to the packet data selected at S98. If theAQM-based packet data drop is to be applied, the operation proceeds toS100. Otherwise, the operation jumps to S102.

If it is determined that the AQM-based packet data drop is to be appliedat S99, the target base station selects the packet data to drop at S100.

At S101, the target base station drops the packet data selected at S100.

At S102, the buffer unit in the target base station forwards the packetdata to a lower layer.

FIG. 30 is a flowchart of packet data processing in the DL communicationat the user equipment.

At S103, the user equipment receives data through the sending/receivingunit.

At S104, the user equipment performs error detection on the data at theMAC layer received at S103. If an error is detected, the operationproceeds to S105. Otherwise, the operation proceeds to S107.

At S105, the user equipment determines whether the resending timer atthe RLC layer for the data from which an error has been detected at S104has expired. If the resending timer has expired, the operation proceedsto S106. Otherwise, the operation returns to S103.

At S106, the user equipment drops the data whose resending timer hasexpired at S105.

At S107, the user equipment makes the received data into packet data asa PDCP SDU (PDCP SDU construction).

At S108, the user equipment determines whether the packet data that isconstructed at S107 has incurred any loss, meaning, any missing part dueto a transmission failure. If the packet data has a loss, the operationproceeds to S109. Otherwise, the operation proceeds to S112.

At S109, when a PDCP SDU yet to be received is detected, the userequipment starts the reordering timer.

At S110, the user equipment waits for the arrival of the PDCP SDU yet tobe received until the reordering timer expires.

At S111, the user equipment determines whether the reordering timer hasexpired. If the reordering timer has not expired, the operation returnsto S110. Otherwise, the operation proceeds to S112.

At S112, the user equipment forwards the packet data to an upper layer.

(10) Fifth Embodiment (a) Overview

In the fifth embodiment, an example of packet data drop operations inthe case where a part of data, or packet data is not successfully sentin the DL communication from the source base station to the userequipment due to a radio error before the handover is described as inthe fourth embodiment.

The outlined data transmission at the user equipment and the source basestation before the handover are same as those described in FIG. 25A andFIG. 25B of the fourth embodiment.

FIG. 31A, FIG. 31B, FIG. 31C and FIG. 31D are diagrams illustrating thedata transmission from the source base station to the target basestation and the data transmission from the target base station to theuser equipment after the handover.

In FIG. 31A, FIG. 31B, FIG. 31C and FIG. 31D, reference numeral 703designates the packet drop control unit that is assumed to be includedin the control unit 215 illustrated in FIG. 13.

In this embodiment, the source base station estimates the transfer delaytime, a delay time that occurs when the packet data is transferred fromthe source base station to the target base station, reflects theestimated transfer delay time on the drop timers for the packet data tobe transferred to the target base station in advance, and then performsthe packet data transfer.

In FIG. 31A, FIG. 31B, FIG. 31C and FIG. 31D, the drop timers for thepacket data 1 to 8 that is stored in the buffer unit 212 a of the sourcebase station after the handover have the values of 80, 70, . . . , 10,respectively.

The source base station sets the drop timers for the packet data withnew values that reflect the estimated transfer delay time, 20 forexample in FIG. 31A and FIG. 31B. In FIG. 31A and FIG. 31B, the droptimers for the packet data 1 to 8 have the new values of 100, 90, . . ., 30, respectively.

The source base station transfers the packet data 2, which was failed intransmission to the user equipment before the handover, to the targetbase station.

The source base station also transfers the packet data 4 to 8, which hasnot been sent before the handover, to the target base station. Here, thesource base station may complete the processing on the packet data thatis being processed in the buffer unit 212 a among the packet data 4 to 8and transfer the packet data with the SNs added or may suspend theprocessing and transfer the packet data without the SNs added.

The packet data 9 and 10 to be stored in the buffer unit 212 b of thetarget base station illustrated in FIG. 31C is the packet data includedin the data received from the upper station.

The target base station takes over the drop timers for the packet dataset in the source base station. Specifically, the drop timers for thepacket data 2 and the packet data 4 to 8 that are stored in the bufferunit 212 b of the target base station have the values of 90, 70, 60, . .. , 30, respectively.

In the fifth embodiment, the target base station may apply the AQM-basedpacket data drop only to the data received from the upper station, ormay apply it to both the data received from the upper station and thedata that has not been forwarded to a lower layer among the datareceived from the source base station.

The buffer unit 108 of the user equipment reorders the packet dataincluded in the data received from the target base station and thealready stored packet data, and then forwards the packet data to anupper layer in the order the packet data sent, or in the order of SNs.In FIG. 31D, for example, the buffer unit of the user equipment forwardsthe packet data in the order of the packet data 2 and the packet data 3to an upper layer.

(b) Takeover of Drop Timer

The drop timer is taken over at a time stamp field in a packet datatransfer protocol header.

The packet data transfer protocol header is the GTP header illustratedin FIG. 32 defined by 3GGP.

In the header part illustrated in FIG. 32: the Version field representsa GTP protocol version; the Protocol Type (PT) represents protocolidentification; the Extension Header flag (E) represents information onthe Next Extension Header field; the Sequence number flag (S) representsinformation on the SN field; the N-PDU Number flag (PN) representsinformation on the N-PDU Number field; the Message Type represents theGTP message type; the Length shows the payload length; the TunnelEndpoint Identifier (TEID) represents information for clarifying theendpoint of the tunnel; the Sequence Number (SN) represents the sequencenumber used for identifying data; the N-PDU Number representsidentification used in adjusting the data transfer; and the NextExtension Header Type represents identification of the extension header.

The Extension Header part in the figure includes information on theextension header such as the Extension Header Length, the ExtensionHeader Content, and the Next Extension Header Type.

FIG. 33 is a diagram illustrating an example of a header used in thepacket data transfer in the embodiment.

The Next Extension Header Type in the header part defines that the PDCPSN is sent in the extension header. The source base station sendsinformation on the drop timers for the transfer packet data included inthe time stamp by defining the Timestamp field in the extension headerpart.

The source base station can also send the abovementioned time stamp onthe SN State Transfer message, for example.

FIG. 34 is a diagram illustrating an example of a case where the sourcebase station sends a time stamp on the SN State Transfer message.

In FIG. 34, the Last-In-Sequence (LIS) is the last PDCP SDU SN numberthat the source base station received in order.

The Bitmap_N field represents whether the source base station has sentthe PDCP SDU whose SN follows the LIS to the user equipment by one bit.

The Timestamp_LIS represents the value of the drop timer for the PDCP SNof the LIS.

Likewise, the Timestamp_X represents the value of the drop timer for thePDCP SDU that the source base station has not correctly sent.

FIG. 35 is a diagram illustrating another example of a case where thesource base station sends a time stamp on the SN State Transfer message.

In FIG. 35, the Missing_N represents the PDCP SN that the source basestation is not successfully sent to the user equipment.

The Timestamp_N represents the value of the drop timer for the PDCP SDUthat the source base station has not correctly sent.

The source base station may limitedly take over the timer for an onlyarbitrarily selected packet instead of taking over the timer for eachpacket. For example, the source base station may transfer only the valueof the drop timer corresponding to the packet that is to be transferredfirst. In that case, the target base station may set its own timervalues for the drop timers corresponding to the other packets as shownin the fourth embodiment with reference to the drop timer values thatare transferred by the source base station.

(c) Drop Timer Setting

Drop timers may be set by the methods below.

1) Timer Value Designating Method

The source base station describes the drop timer value corrected withthe transfer delay time in the Timestamp field.

When the source base station is to transfer the packet data whose droptimer has the value set to 40 to the target base station, for example,the source base station describes the value “40+transfer delay time”(=T0) in the Timestamp field. For the transfer delay time, the valueestimated by the source base station is used here.

The target base station sets the value described in the Timestamp fieldto the value of the drop timer for the corresponding packet data.

The present embodiments may be adapted not to need the source basestation to transfer the packet data in the case where T0≧ the upperlimit of the drop timer.

2) Absolute Time Designating Method

The source base station describes the time to drop the packet data inthe Timestamp field.

When the source base station is to transfer the packet data whose droptimer has the value set to 40 and the upper limit set to 100 to thetarget base station, for example, the source base station describes thevalue “the current time at the source base station+(100−40)” (=T1) inthe Timestamp field.

The target base station compares the time described in the Timestampfield T1 and the current time at the target base station when thecorresponding packet data is transferred thereto T2. When T1≧T2, thetarget base station sets T1 to the value of the drop timer for thecorresponding packet data. When T1≦T2, the target base station drops thecorresponding packet data.

The present embodiments may be adapted not to need the source basestation to transfer the packet data in the case where T1≦T2 is known atthe source base station. For the absolute time, the time synchronizedbetween the base stations by means of the Global Positioning System(GPS) or the time calculated by both of the base stations that recognizethe difference between the time at the source base station and the timeat the target base station may be used.

3) Relative Time Designating Method

The source base station describes the time interval between the currenttime and the time to drop the packet data corrected with the transferdelay time in the Timestamp field.

When the source base station is to transfer the packet data whose droptimer has the value set to 40 and the upper limit set to 100 to thetarget base station, for example, the source base station describes thevalue “(100−40)−transfer delay time” (=T3) in the Timestamp field. Forthe transfer delay time, the value estimated by the source base stationis used here.

When the time T3 described in the Timestamp field≧0, the target basestation sets the value “100−T3” to the value of the drop timer for thecorresponding packet data. When T3≦0, the target base station drops thepacket data. The present embodiments may be adapted not to need thesource base station to transfer the packet data in the case where T3≦0.

(d) Another Example of Drop Timer Setting

The drop timers may be set by means of a sub-frame number in (c) 1)through 3), instead of the Timestamp field.

In LTE, the base station can identify the sub-frame number for eachTransmission Time Interval (TTI). Thus, the value of the drop timer maybe described by means of the sub-frames in such a manner of expressingthe time 40 by 40 sub-frames, for example.

For the sub-frame number, the number synchronized between the basestations or the number calculated by the source base station and thetarget base station that recognize the difference between the sub-framenumbers may be used.

(d) Processing Flow

FIG. 36 is a flowchart of packet data processing in the DL communicationat the source base station.

At S113, the buffer unit in the source base station receives data sentfrom an upper layer.

At S114, the source base station makes the data received by the bufferunit at S113 into packet data as a PDCP SDU (PDCP SDU construction).

At S115, the source base station starts the drop timer for each packetdata constructed at S114.

At S116, the source base station sends the HO Instruction signal to theuser equipment.

After sending the HO Instruction signal at S116, the source base stationdetermines whether to transfer the packet data to the target basestation at S117. If the packet data is to be transferred, the operationproceeds to S118. Otherwise, the operation proceeds to S121.

If the source base station determines to transfer the packet data atS117, the source base station reports the upper limit of the drop timerfor the packet data to transfer to the target base station at S118. Asmentioned above, the upper limit of the timer is reported to the targetbase station at any of “3. HO Request”, “7. SN State Transfer” and “9.HO Completion” illustrated in FIG. 7. Here, it is assumed that the upperlimit is reported by the “7. SN State Transfer” message.

At S119, the source base station estimates the transfer delay time.

At S120, the source base station takes over the drop timer for thepacket data to transfer.

If the source base station determines not to transfer the packet data atS117, the source base station reports the upper limit of the drop timerto the target base station at S121. As mentioned above, the upper limitof the timer is reported to the target base station at any of “3. HORequest”, “7. SN State Transfer” and “9. HO Completion” illustrated inFIG. 7. Here, it is assumed that the upper limit is reported by the “7.SN State Transfer” message.

At S122, the buffer unit in the source base station forwards the packetdata to a lower layer until the radio link is disconnected.

FIG. 37 is a flowchart of packet data processing in the DL communicationat the target base station.

At S123, the buffer unit of the target base station receives the datasent from an upper layer.

At S124, the target base station determines whether the data received atS123 is the data sent from the upper station. If it is the data sentfrom the upper station, the operation proceeds to S125. Otherwise, theoperation proceeds to S126.

If the target base station determines that the data received at S124 isthe data transferred from the source base station, instead of the datasent from the upper station, the target base station obtains the valueof the drop timer to be set for the packet data included in the data atS125.

At S126, the target base station makes the data received at S123 intopacket data as a PDCP SDU (PDCP SDU construction).

At S127, the target base station starts the drop timer for the packetdata constructed at S126. For the data received from the upper station,the target base station sets the upper limit of the drop timer. Theupper limit of the drop timer is set to 100, for example. For the datareceived from the source base station, the target base station sets thevalue of the drop timer obtained at S125.

At S128, the target base station determines whether the drop timer thatis started at S127 has expired for each packet data. If the drop timerhas expired, the operation proceeds to S129. Otherwise, the operationproceeds to S130.

If the drop timer has expired at S128, the target base station drops thepacket data that has been forwarded to a lower layer at S129.

At S130, the target base station selects the packet data received fromthe upper station.

At S131, the target base station determines whether the AQM-based packetdata drop is to be applied to the packet data selected at S130. If theAQM-based packet data drop is to be applied, the operation proceeds toS132. Otherwise, the operation jumps to S134.

If it is determined that the AQM-based packet data drop is to be appliedat S131, the target base station selects the packet data to drop atS132.

At S133, the target base station drops the packet data selected at S132.

At S134, the buffer unit in the target base station forwards the packetdata to a lower layer.

The packet data processing flow in the DL communication at the userequipment is the same as that of the fourth embodiment.

(11) Sixth Embodiment

In the sixth embodiment, as in the fourth embodiment, an example ofpacket data drop operations in the case where a part of data, or packetdata, is not successfully sent in the DL communication from the sourcebase station to the user equipment due to a radio error before thehandover is described.

The outlined data transmission at the user equipment and the source basestation before the handover are same as those described in FIG. 25A andFIG. 25B of the fourth embodiment.

FIG. 38A, FIG. 38B, FIG. 38C and FIG. 38D are diagrams illustrating thedata transmission from the source base station to the target basestation and the data transmission from the target base station to theuser equipment after the handover.

In FIG. 38B and FIG. 38C, reference numeral 703 designates the packetdrop control unit that is assumed to be included in the control unit 215illustrated in FIG. 13.

In this embodiment, the drop timers for the packet data to betransferred from the source base station to the target base station aretaken over as they are. The target base station measures/calculates thetransfer delay time, a delay time that occurs when the packet data istransferred from the source base station to the target base station,reflects the estimated transfer delay time on the drop timers for thepacket data received from the source base station, and newly sets thedrop timers.

In FIG. 38A, the drop timers for the packet data 1 to 8 that is storedin the buffer unit 212 a of the source base station after the handoverhave the values of 80, 70, . . . , 10, respectively.

The source base station transfers the packet data 2, which the sourcebase station failed to send to the user equipment before the handover,to the target base station.

The source base station also transfers the packet data 4 to 8, which hasnot been sent before the handover, to the target base station. Here, thesource base station may complete the processing on the packet data thatis being processed in the buffer unit 212 a among the packet data 4 to 8and transfer the packet data with the SNs added or may suspend theprocessing and transfer the packet data without the SNs added.

The packet data 9 and 10 to be stored in the buffer unit 212 b of thetarget base station illustrated in FIGS. 38B and 38C is the packet dataincluded in the data received from the upper station.

The target base station takes over the drop timers for the packet dataset in the source base station. Specifically, the target base stationsets the drop timers for the packet data 2 and the packet data 4 to 8that are stored in the buffer unit 212 b to 70, 50, 40, . . . , 10,respectively.

The target base station measures/calculates the transfer delay time, 20for example, in the figure, adds the transfer delay time to the droptimers for the packet data that are taken over from the source basestation, and sets the drop timers as the new drop timers. In FIG. 38C,for example, the target base station sets the drop timers for the packetdata 2 and the packet data 4 to 8 to 90, 70, 60, . . . , 30,respectively.

In the sixth embodiment, the target base station may apply the AQM-basedpacket data drop only to the data received from the upper station, ormay apply it to both the data received from the upper station and thedata that has not been forwarded to a lower layer among the datareceived from the source base station.

The buffer unit 108 of the user equipment reorders the packet dataincluded in the data received from the target base station and thealready stored packet data, and then forwards the packet data to anupper layer in the order the packet data sent, or in the order of SNs.In FIG. 38D, for example, the buffer unit 108 of the user equipmentforwards the packet data in the order of the packet data 2 and thepacket data 3 to an upper layer.

The takeover of the drop timers for the packet data transferred from thesource base station to the target base station and the setting of thetaken over drop timers at the target base station may be performed bymeans of the time stamp and the sub-frame number as described in thefifth embodiment by the timer value designating method, the absolutetime designating method, and the relative time designating method.

The source base station, however, does not perform correction with thetransfer delay time in the timer value designating method and therelative time designating method described in the fifth embodiment. Inthose cases, the target base station corrects the values of the droptimers with the transfer delay time measured by itself and sets thevalues.

The source base station may limitedly take over the timer for an onlyarbitrarily selected packet instead of taking over the timer for eachpacket. For example, the source base station may transfer only the valueof the drop timer corresponding to the packet that is to be transferredfirst. In that case, the target base station may set its own timervalues for the drop timers corresponding to the other packets as shownin the fourth embodiment with reference to the drop timer values thatare transferred by the source base station.

(12) Others

The value of the drop timer that is taken over from the source basestation to the target base station upon packet data transmission may bea value corresponding to the SN of the packet data or a time asdescribed in the first to sixth embodiments. For example, the value ofthe drop timer is reported from the source base station to the targetbase station as PDCP SDU SN i×Ni, where Ni is the coefficient for thePDCP SDU SN i.

Accordingly, it is possible to reduce the transmission delay byproviding an effective method for dropping packet data during handoverfor the user equipment and the base station.

According to an aspect of the invention a radio communication deviceincludes a control unit configured for dropping packet data, the datawith identification for dropping added, based on the identification fordropping or a method for dropping packet data without based on theidentification according to the state of the data, wherein the data isdropped based on the identification or not based on the identificationfor dropping according to an interface through which the data istransmitted.

According to an aspect of the invention a radio communication deviceincludes a control unit configured for dropping packet data, the datawith identification for dropping added, based on the identification fordropping or a method for dropping packet data without based on theidentification according to the state of the data, wherein theidentification for dropping is distinguished before and after ahandover, if required, and the dropping is kept when the handoveroccurs.

According to an aspect of the invention a control method includesperforming a method for dropping packet data, the data withidentification for dropping added, based on the identification fordropping or a method for dropping packet data without based on theidentification for dropping according to the state of the data, whereina drop time is taken over by any of means for taking over a timer value,means for taking over a time to perform the dropping, and means fortaking over a time left until the dropping, and the drop time is set byfurther taking account of an estimated transfer delay time for the datawhen the handover occurs.

According to an aspect of the invention a control method includesperforming a method for dropping packet data, the data withidentification for dropping added, based on the identification fordropping or a method for dropping packet data without based on theidentification for dropping according to the state of the data, whereinif a drop time is earlier than the current time, the data is droppedwithout being transferred, or the data is dropped immediately after thedata is transferred when the handover occurs.

According to an aspect of the invention a control method includesperforming a method for dropping packet data, the data withidentification for dropping added, based on the identification fordropping or a method for dropping packet data without based on theidentification for dropping according to the state of the data, whereina drop time or a time left until the dropping is taken over as describedin a data transfer protocol header when the handover occurs.

According to an aspect of the invention a control method includesperforming a method for dropping packet data, the data withidentification for dropping added, based on the identification fordropping or a method for dropping packet data without based on theidentification for dropping according to the state of the data, whereinif a station to manage the dropping is changed, the upper limit of atimer is taken over by a control signal from an upper station ordirectly taking over by a control signal exchanged between stations whenthe handover occurs.

According to an aspect of the invention a control method includesperforming a method for dropping packet data, the data withidentification for dropping added, based on the identification fordropping or a method for dropping packet data without based on theidentification for dropping according to the state of the data, whereinif a station to manage the dropping is changed, the upper limit of atimer is taken over by a control signal from an upper station ordirectly taking over by a control signal exchanged between stations whenthe handover occurs, and if the identification for dropping isdistinguished when the handover occurs, sending/receiving a controlsignal for sharing the distinction by a base station and a userequipment between the base station and the user equipment.

All examples and conditional language recited herein are intended forpedagogical purposes to aid the reader in understanding the inventionand the concepts contributed by the inventor to furthering the art, andare to be construed as being without limitation to such specificallyrecited examples and conditions, nor does the organization of suchexamples in the specification relate to a showing of the superiority andinferiority of the invention. Although the embodiments of the presentinventions have been described in detail, it should be understood thatthe various changes, substitutions, and alterations could be made heretowithout departing from the spirit and scope of the invention.

1. A mobile communication system, comprising: a buffer storing packetdata to be sent; and a controller configured for discarding the packetdata according to a value of a timer corresponding to the packet data,maintaining the value of the timer corresponding to the packet data whena handover is performed without restarting or resetting the value of thetimer, wherein the discarding further includes discarding thecorresponding packet data when the value of the timer reaches a givenvalue.
 2. The system as defined in claim 1, wherein the packet data is aservice data unit (SDU) in a Packet Data Convergence Protocol (PDCP). 3.A mobile communication system, comprising: a controller configured fordiscarding stored packet data according to a value of a timercorresponding to the packet data; and a transmitter configured fortransferring the packet data from a source base station to a target basestation and transferring the value of the timer corresponding to thepacket data to be transferred when a handover is performed; wherein thetarget base station sets a new value of the timer for the transferredpacket data with the transferred value of the timer or the transferredvalue of the timer corrected with a transfer delay time, and when thevalue of the timer, discards the corresponding packet data.
 4. Thesystem according to claim 4, wherein the source base station transfersthe value of the timer by using a packet data reception state reportingprotocol or a time stamp field included in a packet data transferprotocol header.
 5. The system as defined in claim 3, wherein the packetdata is a service data unit (SDU) in a Packet Data Convergence Protocol(PDCP).
 6. A mobile communication system comprising: a buffer configuredto store packet data to be sent; a transmitter for transferring thepacket data from a source base station to a target base station andtransferring a value of a timer corresponding to the packet data to betransferred when a handover is performed; and a controller configuredfor setting a new value of the timer for the transferred packet datawith the transferred value of the timer or the transferred value of thetimer corrected with a transfer delay time, and for discarding thecorresponding packet data when the value of the timer, wherein themobile communication system discards the packet data stored in thebuffer unit according to the value of the timer corresponding to thepacket data.
 7. The mobile communication system according to claim 6,wherein the source base station transfers the value of the timer byusing a packet data reception state reporting protocol or a time stampfield included in a packet data transfer protocol header.
 8. The systemas defined in claim 6, wherein the packet data is a service data unit(SDU) in a Packet Data Convergence Protocol (PDCP).
 9. A mobilecommunication system comprising: a buffer configured to store packetdata to be sent; a transmitter for transferring the packet data from asource base station to a target base station, and correcting a value ofa timer corresponding to the packet data to be transferred with atransfer delay time and then transferring the value of the timer when ahandover is performed; and a controller configured for setting a newvalue of the timer for the corresponding packet data with the correctedvalue of the timer transferred thereto, and for discarding thecorresponding packet data when the value of the timer, wherein themobile communication system discards the packet data stored in thebuffer unit according to a value of the timer corresponding to thepacket data.
 10. The mobile communication system as defined in claim 9,wherein the packet data is a service data unit (SDU) in a Packet DataConvergence Protocol (PDCP).